Convert to Low Cont. Speed furnace?

Hi.. I have a Carrier 58MCA furnace (about 3yrs old). With the fan on continuous, it runs at high speed only (same as heating). I would like to change something so that it runs slower (quieter) at continuous fan and back to faster when heat is called for.
An HVAC company has quoted me $1200 to make it "variable speed".
There were 4 models of this furnace series--each with a different control board. The top one was "Variable speed" but the two middle ones were simpler--offering a low cont. speed option (mine is the lowest model--no low speed).
Do I need to change the motor, or just the control board? Not necessary to make it "variable speed" -- just low cont. fan....
Any Carrier experts out there??
Peace, Dan
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